1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus for drilling with casing. Particularly, the present invention relates to methods and apparatus for reducing drilling vibration while drilling with casing. Additionally, the present invention relates to apparatus and methods for manufacturing a vibration damper.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed in a formation using a drill bit that is urged downwardly at a lower end of a drill string. To drill within the wellbore to a target depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of metal pipe called casing. The casing string liner is temporarily hung from the surface of the well.
The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface to a designated depth. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. Using apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, one conventional method of completing a well includes drilling to a first designated depth with a drill bit on a drill string. Then, the drill string is removed and a first string of casing is run into the wellbore and set in the drilled out portion of the wellbore. Cement is circulated into the annulus behind the casing string and allowed to cure. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the second string of casing in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to a desired depth. Therefore, two run-ins into the wellbore are required per casing string to set the casing into the wellbore.
Because of the two run-in requirement, the traditional method of using the drillstring (pipe with drill bit on bottom) to form a wellbore is time consuming and expensive. The time required to remove the drilling string as the wellbore is extended results in an increase of operational time and costs. For example, an offshore drilling platform may rent for hundreds of thousands of dollars a day. Accordingly, reducing drilling time by even an hour may significantly reduce drilling costs.
Another method for performing well completion operations involves drilling with casing. In contrast to drilling with drill pipe and then setting the casing, drilling with casing entails running a casing string into the wellbore with a drill bit attached. The drill bit is operated by rotation of the casing string from the surface of the wellbore. Once the borehole is formed, the attached casing string is cemented in the borehole. The subsequent borehole may be drilled by a second casing having a second drill bit at a lower end thereof. The second casing string may be operated to drill through the drill bit of the previous casing string. In this respect, this method requires only one run-in into the wellbore per casing string that is set into the wellbore.
While drilling with casing provides an efficient system for wellbore completion, the system does have its drawbacks. For example, drilling with casing is sometimes more prone to drilling vibrations than the conventional drill pipe string. Excessive drilling vibration is a cause of premature failure or wear of drilling components and drilling inefficiency. Two common forms of drilling vibration include backwards whirl and stick slip vibration. Backwards whirl occurs due to lateral vibrations caused by the drillstring eccentricity, which may lead to centripetal forces during rotation. Stick slip vibration occurs due to torsional vibrations caused by nonlinear interaction between the drillstring and borehole wall. Slip stick vibration is characterized by alternating stops and intervals of large angular velocity.
Drilling vibration may occur more frequently in drilling with casing than conventional drilling. This is because drilling casing has a larger outer diameter than drill pipes. As a result of the smaller clearance, the potential for interaction between the drilling casing and the existing set casing is increased. As the drilling casing is rotated to the right, it can backwards whirl to the left along the ID of the set casing. The resultant centripetal forces are very high. This centripetal force can sometimes cause galling between the drilling-casing couplings and the set casing ID. The end result is an increase in drilling vibration and torque, sometimes to unacceptable levels.
Therefore, there is a need for apparatus and methods to reduce drilling vibration while drilling with casing. There is a further need for apparatus and methods to reduce friction between a drilling casing and an existing casing.